Madison, Wisconsin (March 5, 2014)--While most agricultural scientists focus on what herbicides do within crop fields, agroecologist Franklin Egan has been looking outside the field to the impacts of these chemicals on wild plants. In research he did for his doctorate, Egan and his Penn State University colleagues bring attention to the wild plant diversity within agricultural landscapes and the potential effects of herbicides on it.

It may seem like a strange thing to study in a discipline where crop production is paramount, but new developments are making it increasingly relevant, says Egan, now a postdoc with USDA-ARS. Even in the world’s most intensively farmed regions, research is finding that hedgerows, old fields, woodlots, and other bits of native habitat can be “important reservoirs” of plant diversity, he explains. Wild plants, in turn, provide overwintering habitat and pollen and nectar for beneficial insects. These include pollinators like native bees, and lady beetles and other predators of crop pests.

In fact, the amount of these remnant habitats has been linked to the magnitude within farmlands of “ecosystem services” such as pollination and pest control.

“Basically, these habitats and the plants living in them do a lot of valuable work,” Egan says. So “whenever we can find ways to conserve plants within the agricultural landscape I think it’s a valuable strategy,” he adds, especially since agriculture covers so much of the planet.

In the meantime, corn and soybean varieties engineered to tolerate the herbicides 2,4-D and dicamba are poised to enter the market in the next few years, leading to what many predict will be a dramatic rise in the spraying of these chemicals. The issue is these herbicides are among those most often implicated in drift complaints—situations where herbicides float off target and cause unintended harm to sensitive species in neighboring fields and habitats.

To reduce this possibility, companies are now commercializing ultra-low volatility formulations. But the potential effects on wild plant communities are still a concern, says Egan. In particular, both 2,4-D and dicamba readily kill wildflowers and other “broadleaf” species, while grasses are more tolerant.

“So if small amounts of these compounds from herbicide drift end up in field margins or grasslands that are providing a lot of habitat for wildflowers, we might see a shift in those areas to grasses, which would provide fewer floral resources” to insects, he says.

That’s the hypothesis he and his collaborators tested in a study published in Agriculture, Ecosystems and Environment in early February. Experiments were performed at two sites in Pennsylvania: a field margin containing both grasses and wildflowers next to an alfalfa field; and an old field that was planted in wildflowers as part of a Natural Resources Conservation Program pilot program to establish pollinator habitat. During the two-year study, the researchers applied dicamba at low doses designed to mimic herbicide drift. They then monitored changes in plant and insect communities.

In the field margin plots, the community responded much like the scientists expected. “We saw a clear trend of increasing grass dominance at the expense of cover in broadleaf plants,” Egan says. The shift also occurred at doses as low as 1% of dicamba’s field application, or label, rate—a level on par with herbicide spray drift.

The dicamba applications also correlated with declines in two aphid species and the potato leaf hopper, while an alfalfa pest, clover root curculio, grew more abundant.

Something the group didn’t observe was a drop in pollinators, which might be expected following the loss of wildflowers. But pollinators are known to fly long distances, Egan explains. “So it could be that at the scale of the small, replicated field experiment we weren’t able to detect that effect.”

What the researchers also didn’t detect was much of an effect of dicamba in the old field. Only when they began applying dicamba’s field label rate, or a full dose, did they begin to see any change. Why the plant communities in the field margin and the old field responded so differently isn’t clear, Egan says. One possibility is that intense sun and drought conditions in the old field limited dicamba’s effectiveness.

What is apparent from the findings is the need to take care as 2,4-D- and dicamba-resistant crops enter the marketplace. “The variability from site to site, the uncertainty, I think underscores the need to be very cautious with these compounds,” Egan says. “We certainly have a lot of research on best management practices to limit drift, and we need to be focused on using those practices to the best of our abilities.”

At the same time, he doesn’t want people to fixate on herbicides as the cause of biodiversity loss in farmlands (a second paper in Environmental Toxicology and Chemistry addresses this). There are many other reasons why plant diversity has dropped in farm landscapes over the past several decades, including simplified crop rotations, segregation of crops and livestock, and—especially—the clearing of woodlots, wetlands, and grasslands to make way for bigger fields.

“Herbicide pollution is probably significant in some cases,” he says. “But it’s clearly the case that preserving these habitats of hedgerows, woodlots, and pastures is really important for plant conservation.”